1. Field of the Invention
This invention relates to a water-based magenta ink composition suited for ink-jet recording, and an ink-jet recording process making use of it.
2. Description of the Related Art
Ink-jet recording is a system in which ink droplets are formed by ink ejection methods of various types (e.g., a system utilizing electrostatic attraction, a system in which mechanical vibration or displacement is imparted to ink by the use of a piezoelectric device, and a system in which ink is heated to cause it to bubble and a pressure produced when it bubbles is utilized), and part or the whole of ink droplets is caused to adhere to recording mediums such as paper to make a record.
As ink compositions used in such ink-jet recording systems, pigment ink compositions or dye ink compositions prepared by dissolving or dispersing water-soluble dyes or pigments of various colors in water or mixed solvents of water and water-soluble organic solvents are known in the art.
Ink compositions for ink-jet recording are required to have various performances such that they cause no clogging at nozzles and in ink channels of the head of a recording apparatus, such that they cause no changes in properties and no deposition of solid matter during their storage, such that they give recorded images having sharp color tones and sufficiently high densities, such that they enable recording without limitation on the type of recording mediums, such that they have a high rate of fixing to recording mediums, such that they have excellent water resistance, weatherability, solvent resistance and abrasion resistance, such that they give recorded images having a good resolution, and such that they have values of physical properties such as viscosity and surface tension within proper ranges. In particular, what is important is the liquid stability that the ink does not cause clogging of, and deposits at, nozzles and ink channels of the recording apparatus.
Pigment ink compositions have a superior weatherability, but have a poor liquid stability since the pigment is dispersed in a solvent. During their storage for a long period of time, the dispersion of pigment may become unstable to cause agglomeration. Hence, they tend to cause clogging at the head nozzles or the like of the ink-jet recording apparatus. Accordingly, it is attempted to make improvement by regulating the constitution of pigment ink compositions as disclosed in Japanese Patent Application Laid-open No. 1-204979, or by controlling the pH of water-based pigment ink compositions as disclosed in Japanese Patent Application Laid-open No. 64-6074. They, however, can not well endure the use as ink-jet recording inks.
Dye ink compositions, though having weatherability inferior to the pigment ink compositions, do not tend to cause clogging at nozzles and in ink channels of the head of the recording apparatus, and are widely used as ink-jet recording inks.
Many of such dye ink compositions are available as water-based ink compositions prepared by dissolving water-soluble dyes having various chemical structures, in water or mixed solvents of water and water-soluble organic solvents.
Such water-based dye ink compositions may cause a decrease in solubility of dyes to cause deposits of dyes, when the quality of liquid is nearly neutral or acidic. On the other hand, when strongly alkaline, they may cause deformation of members used in the ink-jet recording apparatus, in particular, members made of organic materials, or the organic materials constituting the members may dissolve into the ink, undesirably. They may also be harmful to human bodies, e.g., may attack the cornea, and hence, in order to adjust the pH of inks to the weakly alkaline side, some of them employ pH adjusters such as inorganic salts such as KOH and NaOH or inorganic salt components having a buffer action.
However, such water-based dye ink compositions containing inorganic salts tend to cause the inorganic salt components to deposit at the head nozzles of ink-jet printers, to cause clogging. When the ink compositions are used over a long period of time, the addition of the pH adjuster becomes less effective, so that the pH may become lower and the dye may become deposited to make the liquid stability poor.
Meanwhile, when full-color recorded images are reproduced using water-based dye ink compositions, four primary color inks, i.e., yellow ink, magenta ink, cyan ink and additionally black ink are used, and color tones of recorded images are determined by subtractive color mixing. In order to give recorded images having a sharp color tone as one of performances of the above inks, these inks are also required to form ideal primary colors of yellow, magenta, cyan and black whose hues each have no other color tint.
In order to form ideal hues, dyes must be selected. In particular, in the case of magenta ink, only few dyes can reproduce magenta color, and Color Index Number Acid Red 52 (hereinafter often "C.I. Acid Red 52") is usually used. This dye can form a good magenta color, but, when present in ink compositions, may cause a great change in pH as a result of long-term storage, resulting in a poor liquid stability.